Introduction to Soil Chemistry

plants 51
Rhizosphere
Figure 3.4. Illustration of the concept of the rhizosphere as an area around plant roots.
One well-studied root type is that of the legumes, which are infected by a
microorganism called rhizobia, resulting in the formation of nodules on the
roots. A symbiotic relationship exists between the rhizobia and the legume
wherein rhizobia supply fixed nitrogen to plants and plants provide carbohydrates,
an energy source, to the rhizobia. Fixed nitrogen is any nitrogen atom
bonded to another atom other than nitrogen (N 2), as in NH 3, NO 3 - ,
(NH 2) 2CO; in rhizobia the nitrogen compound produced is ammonia, which is
used by plants to produce amino acids.
It is to be expected that soil with a sod cover, that is, with thick grass and
roots, will have characteristics different from those of a soil on which few or
no plants are growing. These differences will be important in analyzing soil for
components of concern.
There are many types of roots, ranging from thick fibrous, deep tap, shallow,
tubers, all in one plant community. Some roots explore the soil to significant
depth (i.e., as much as 250cm deep), while others are shallow (i.e., only 25cm
deep). Different rooting depths are found in all plant types; grasses, legumes,
shrubs, and trees. Each root type will contribute its own unique exudates and
characteristics to its unique volume of soil and the associated soil solution.
Plant roots respire, taking in oxygen and giving off carbon dioxide. This is
a simple but essential process, and most land plants die if their oxygen source
is interrupted for even a short period of time. There are, however, some plants,
including crops, which grow with their roots submerged in water, that is, under
anaerobic and reducing conditions. Cyprus is an example of such a tree, while
rice is an example of a crop. These plants have developed a vascular system
that conducts oxygen to the roots, allowing them to function. However, it is
important to note that the environment of these roots is very different from
that encountered by roots in unsaturated soil. For instance, anaerobic con